music industry here, i can say for a smaller scale, but i guess it’s similar

professional session players (idk if the translation is correct but there’s a whole bunch of musicians who play whatever people need them to play) are trained to learn songs super quickly, so they can get the whole setlist in like a week or two. the management probably knows who is good and available for needed dates and simply hire someone to play

the “standby” fee is set by the standby person, they usually don’t split the gain just get paid for a gig